code: def backward(self, indexes, gy):
      x = self.get_retained_inputs()
      return BatchL2NormSquaredGrad().apply((x[0], gy[0]))
  
file_path: chainer/chainer/functions/math/batch_l2_norm_squared.py
imports: import six
  import chainer
  from chainer.backends import cuda
  from chainer import function_node
  from chainer.functions.math import sum
  from chainer.utils import type_check
label_desc: Use image node_id 4 for calling the BatchL2NormSquared obj's underlying member method code with example usage: obj.backward(indexes, gy) and returns: BatchL2NormSquaredGrad